Generalized free energy and thermodynamic phases of black holes in the gauged Kaluza-Klein theory

Abstract: In the context of the generalized (off-shell) free energy, we explore the phase emergence and corresponding phase transitions of charged dilaton $\text{AdS}$ black holes in the gauged Kaluza-Klein (KK) theory where the KK vector field is gauged such that the fermionic fields are charged under the U(1)$_{\text{KK}}$ gauge group. The black hole solutions are asymptotic to the AdS$_D$ geometry and can be realized as the dimensional reduction of the gauged supergravities on the compact internal manifolds, leading to the restriction as $4\leq D\leq 7$. By studying the behavior of the generalized free energy under the change of the ensemble temperature, we determine the thermodynamic phases and the corresponding phase transitions of black holes. This is confirmed by investigating the heat capacity at the constant pressure and the on-shell free energy. In the canonical ensemble, the thermodynamics of black holes can be classified into three different classes as follows: (i) $D=4$, (ii) $D=5$, and (iii) $D=6,7$. Whereas, in the grand canonical ensemble, the thermodynamics of black holes is independent of the number of spacetime dimensions and the pressure, but depends on the chemical potential $\Phi$. The thermodynamic behavior of black holes can be classified into three different classes as follows: (i) $\Phi<1$, (ii) $\Phi>1$, and (iii) $\Phi=1$.